1. Field of The Invention
This invention relates to photographic apparatus, and particularly to a photographic apparatus having an exposure control system that compensates for differences in background brightness.
2. Prior Developments
Pathologists, and others in the scientific area, often are required to take photographs of surgical specimens, charts and Graphs. One of the photographic techniques involves the use of a 35 mm film recorder. The recorder comprises a rectangular light box (cabinet) having an interior light source. A plate glass panel is placed over a colored plastic sheet to form the top surface of the light box; the specimen or object to be photographed is placed on the glass panel.
In many cases the object is illuminated by two or more overhead lamps carried by bracket arms attached to side surfaces of the light box. The lamps are targeted onto the object from different angles, to eliminate shadows that might degrade the picture clarity.
The camera is located above the light box so as to have a downwardly directed view of the object to be photographed. Support for the camera is provided by an upstanding vertical column affixed to the back wall of the light box; the camera is mounted on an arm that rides up and down on the column, whereby the camera field of view can be adjusted or changed in accordance with different size specimens (objects). A motor can be provided on the arm to move the arm and associated camera up or down automatically (i.e. by remote control). The automatic camera system is advantageous in that it can be used by pathologists and technicians having limited photographic skills.
The overhead location of the camera makes it difficult for a person to view the object through the camera view finder. However a viewing system has been devised for viewing and focusing the scene, without requiring the person to look through the camera. As shown in U.S. Pat. No. 4,899,188 to M. Lecover, the camera can be equipped with a built-in recticle. An overhead light source can be aligned with the camera eyepiece to direct light through the camera onto the object and background plate. The image of the recticle is superimposed on the scene to indicate to the person controlling the apparatus the field of view and the focus of the camera lens (i.e. the sharpness or bluriness of the recticle image indicates the need for focus adjustment.
Using the recticle image as a guide, the person can adjust the camera position (up or down) and the lens-film spacing to obtain a relatively sharp picture having a sufficiently sized image of the specimen.
Another method to format and focus the camera involves the use of a video camera and video monitor. The video camera is mounted above the camera to take a picture through the camera eyepiece and camera optical system; the built-in recticle is imaged on the picture, such that a person observing the picture on the video monitor can format and focus the 35 mm camera. The 35 mm camera is operated to take a picture when the image is the correct size and sharply focused, as determined by the video camera-video monitor system.
The above described systems enable the person to format and focus the 35 mm camera on the specimen without the necessity for the person to look through the viewfinder of the camera.
In many cases the person will have to photograph specimens, charts, graphs, statistical tables, etc., against a range of different backgrounds, e.g white paper, darker paper, or merely the glass support panel. Variation in the brightnes, or dimness, of the background creates a problem for the automatic camera especially the exposure control in the camera.
An automatic camera is usually programmed to produce a rendering of an average scene. When the subject is printed, or mounted, on white paper the automatic camera will tend to underexpose the picture, i.e. allow insufficient light to impinge on the film. The resulting photograph may portray the white background as a muddy gray coloration.
With the use of an exposure meter, experienced photographers can compensate for this situation; i.e. by taking an exposure meter reading of the scene to be photographed, and changing the exposure setting on the camera to an overexposure condition. This will cause the white background to appear white in the photograph.
It is possible, but difficult, to accomplish the over-exposure condition in an automatic camera. One way to achieve overexposure is to place a gray card over the subject. The exposure lock button on the camera is held down while the gray card is removed, and the shutter switch is actuated along with release of the exposure lock. This locks in the overexposure, so that the photograph is taken with an artificially greater exposure, i.e. a wider opening of the iris aperture.
The above procedure is difficult for a person having only rudimentary photographic experience to accomplish. Also, the procedure is cumbersome, especially when photographing a large subject wherein the camera may be elevated to a point near the upper end of the camera support column; it may be difficult for the technician to reach the exposure lock on the camera while making the necessary adjustments.